High density polyethylene is used to produce high tenacity fibers such as for ropes, fishing nets, and the like. Such high density polyethylene is required to have properties such as high stretchability and high tenacity.
In the above-mentioned fiber products, it is known that a narrower molecular weight distribution of the high density polyethylene leads to better mechanical properties. That is, when the molecular weight distribution of the high density polyethylene is narrow, the stretching ratio becomes high, and high tenacity can be obtained by high stretchability. However, if the molecular weight distribution of the high density polyethylene is too narrow, there is a problem that processability thereof becomes very inferior.
On the other hand, in the method for producing polyethylene using a Ziegler-Natta catalyst that is commercially widely used in the art, it is difficult to obtain a polymer having a uniform molecular weight, and the molecular weight distribution thereof is wide.
Although polyethylene having a wide molecular weight distribution has an advantage of good processability, there is a disadvantage that its mechanical properties are lowered, a low molecular weight portion is eluted during processing, and inherent physical properties of the resin are deteriorated.
In order to solve these problems, a method for preparing a polyethylene having a narrow molecular weight distribution using a metallocene catalyst system is proposed. However, in order to apply it to a conventional commercial process such as a slurry process and a gas-phase process, the metallocene must be supported on an appropriate support. In the case of the supported metallocene catalysts used so far, there was a disadvantage that the molecular weight distribution becomes broad and the catalytic activity is lowered.
Moreover, the conventional method has a limitation in the amount of aluminoxane which is a cocatalyst for increasing the catalytic activity, and has a disadvantage that the catalyst cost is greatly increased due to an expensive cocatalyst. In addition, in the case of metallocene catalysts, the hydrogen reactivity is high and the beta-hydrogen elimination reaction predominates, and thus there is a limit to increase the molecular weight. Therefore, when the conventional method is applied, there was a problem that it is difficult to apply it to a high-rigidity resin having a high molecular weight and a narrow molecular weight distribution, that is, a resin for a fiber.